Power tongs are common in well drilling operations for rotating cylindrical drill pipe, casing and the like to thread lengths together. Such tongs can have a closed throat, such that the pipe must enter the tongs through the hole in the middle, or can have an open throat, allowing the pipe to enter the tongs from the side.
Both types of tongs typically include top and bottom tong plates that are fastened together to form a tong housing with a central housing aperture. An open throat may lead to the central housing aperture, and typically a door will be provided to close the throat on the housing. A drive mechanism is mounted in the tong housing and rotates a ring gear. In open throat tongs, where the ring gear has an open throat leading to a central ring aperture, the drive typically comprises a drive motor rotating a pair of substantially horizontal drive gears that engage and rotate the ring gear. The drive gears are arranged so that one is engaging and rotating the ring gear while the open throat rotates past the other. In both open and closed throat tongs, the ring gear is typically held in position during rotation by rollers rotatably fastened between the top and bottom tong plates and bearing against the outer periphery of the ring gear teeth.
A pivoting jaw mechanism is mounted on a cage which rotates with respect to the tong housing to move the jaws into gripping engagement with the pipe. The cage typically comprises top and bottom cage plates located above and below the ring gear and defining a central cage aperture in the middle. The cage may have an open throat leading to the central cage aperture.
In closed throat type power tongs, an end of the pipe is moved through the central aperture and then the pipe moves up and down in the aperture as joints are threaded together or taken apart.
Open throat tongs are moved into position by aligning the open throats on the tong housing, ring gear, and cage and passing the pipe through the aligned open throats into the central aperture. Typically a door closes the open throat on the tong housing to prevent the pipe from accidentally escaping from the tongs.
In either type tongs, when the drive begins to rotate the ring gear, the cage rotates through several degrees with respect to the ring gear and the jaws move into gripping engagement with the pipe. Once the jaws are engaged, the cage and ring gear then rotate together, with respect to the tong housing.
In a typical set of power tongs, each of a pair of jaws is pivotally mounted by a bolt to the top and bottom cage plates. A jaw roller attached to the jaw rests in a valley on the inner periphery of the ring gear when the jaws are in an open position. When the ring gear is rotated by the drive gears, the cage remains substantially stationary, commonly held by the friction of a brake, and the jaw roller thus moves out of the valley and up a slope on the inner periphery of the ring gear and pivots the jaw into engagement with the pipe, at which point the cage and ring gear rotate together with the jaws gripping and rotating the pipe as well. The friction of the brake is overcome by the force of the rotating ring gear engaging the jaws and thus the cage. Reversing the direction of rotation of the ring gear releases the jaws and further reverse rotation will cause the jaws to re-grip the pipe and rotate same in the reverse direction.
The orientation of the parts to allow the required movements relative to each other conventionally requires that cage bolts and spacers, or equivalent fasteners, holding the top and bottom cage plates together pass through the aperture inside the inner periphery of the ring gear. Commonly different sets of jaws are provided for a tong assembly to allow for use on pipes with a range of diameters. The range of jaw sizes that can be accommodated by any given set of power tongs is limited because the cage bolts interfere with the operation of the jaws. Moving these cage bolts and spacers out of the aperture would allow for a wider throat in open throat tongs, and would, with either type, accommodate a wider range of jaws, thus increasing the sizes of pipe that the tongs would service, and decreasing the need for a separate set of power tongs.